For most homes, heating is the largest energy cost by a wide margin, often more than everything electrical put together. That also makes it where the biggest savings live.
Turn it down a touch
The single cheapest saving is the thermostat. Dropping the temperature by one degree cuts a noticeable slice off a heating bill and is barely perceptible day to day, especially with a jumper on. Heating rooms you are not using, and heating the house while everyone is out or asleep, are the other easy wins; a timer or a smart thermostat handles both without you thinking about it.
Stop the heat escaping
Heat you have paid for leaking straight out is the real waste. Draught-proofing doors, windows, letterboxes and unused chimneys is cheap and quick and makes a cold house feel warmer at once. Insulation, lofts first, then walls, costs more up front but is the highest-value improvement most homes can make, paying back over a few winters and then saving for decades.
Get more from the boiler
A few free tweaks help a gas boiler run more efficiently: bleeding radiators so they heat fully, keeping them clear of furniture, and on a condensing combi, turning the flow temperature down so it actually condenses. None of these costs anything, and together they squeeze more warmth from the same gas.
In order of value
Roughly: turn it down and time it, then stop the draughts, then insulate, then fine-tune the boiler. Do them in that order and you spend the least for the most, rather than splashing out on kit before the cheap basics are done.